Process for coating high temperature alloys



United States Patent 3,200,015 PROCESS FOR COATENG HIGH TEMPERA- T URE ALLOYS Urban E. Kuntz, East Hartford, Conn, assignor to United Aircraft Corporation, East Hartford, Conn, a corporation of Delaware No Drawing. Filed Sept. 10, 1962, Ser. No. 222,663

19 Claims. (Cl. 148-63) treating nickel base refractory alloys, cobalt base refractory alloys and columbium base alloys.

In recent years, nickel base refractory alloys, cobalt base refractory alloys, and columbium base alloys have been developed that possesses superior stress-rupture strength at high temperatures. Nickel and cobalt base refractory alloys, are so-called superalloys, have been developed that will retain satisfactory stress-rupture strength at temperatures up to about 1800 F., and columbium base alloys have more recently been developed that exhibit superior stress-rupture strength at temperatures up to as high as 2500 F. Such alloys are particularly useful as blades and vanes for gas turbines, especially jet aircraft engines. The principal problem with such alloys is that that are subjected to increase attack by oxidation and contamination at high temperatures and will eventually fail if not protected by suitable coating.

Columbium base alloys are particularly vulnerable to oxidation at high temperatures and will fail rapidly if not protected by a suitable coating.

According to the present invention there has been discovered a new and improved process for coating metals and alloys of the type described with prolytic silicon nitride to render the metals and alloys resistant to oxidation and corrosion in air at high temperatures.

Among the objects of the present invention are to provide improvements in coating metals and alloys which are resistant to temperatures above 875 C., with pyrolytic silicon nitride.

Other more specific objects of this invention are to provide a process for depositing a continuous pyrolytic silicon nitride coating on high-temperature nickel base refractory alloys, cobalt base refractory alloys and columbium base alloys. This coating gives the metals and alloys described significant resistance to oxidation and contamination in air at elevated temperatures up to approximate- ,ly 1600" C.

Among the further objects of this invention are to provide a continuous coating of pyrolytic silicon nitride on nickel base refractory alloys, cobalt base refractory alloys,

. and columbium base alloys for use as blades in gas turlearned by practice of the inventon, the objects and advantages being realized and obtained by means of the 3,20050115 Patented Aug. 10, 1965 ice improved coating is achieved by first treating the metals and alloys at a temperature in excess of about 875 C. with a gaseous admixture comprising silicon halide and anhydrous ammonia.

If desired, the metal base, and especially the nickel base refractory alloys and cobalt base refractory alloys, may first be treated with air or oxygen at elevated temperatures, e.g., 850 to 950 C. to produce an oxide precoat.

According to another more specific and preferred embodiment of this invention, an improved coating of pyrolytic silicon nitride is obtained on nickel base refractory alloys, cobalt base refractory alloys, and columbium base alloys by contacting the metal or alloy base at a temperature of 870 to 1150 C. with a first gaseous mixture com prising silicon halide and nitrogen and then with a second gaseous mixture of silicon halide and anhydrous ammonia.

The prolytic silicon nitride coating of this invention approximates theoretical crystallographic density and is characterized by translucency, hardness, chemical inertness, oxidation and corrosion resistance, high emissivity, low thermal conductivity, wear resistance and nonporosity. The coating is also a good electrical insulator.

The invention will be described with reference to the metals and alloys particularly referred to hereinabove.

Broadly described, the process of this invention for forming a coating of pyrolytic silicon nitride on a structural member fabricated fnom nickel base refractory alloys, cobalt base refractory alloys, and columbium base alloys includes within its scope the steps of heating the structural member to be coated to a temperature of about 875 to 1150 C., preferably between about 900 and 1100 C., passing a gaseous mixture containing a metal halide, preferably silicon tetrafluoride, and nitrogen over the hot surface for several hours to precoat or nitride the surface of the metal, and then passing a gaseous mixture containing a silicon halide, preferably silicon tetrafluoride, and anhydrous ammonia over the hot surface until the desired depth of deposit of silicon nitride is obtained.

The reaction has been carried out in vacuo.

Rather than conducting the reaction in vacuo, a carrier gas may be employed and the reaction conducted at ordinary atmospheric pressure or even super-atmospheric pressure. Suitable carrier gases include nitrogen and the noble gases, such as neon, krypton, argon and the like.

Regardless whether vacuum or an inert gas is employed, the partial pressure of the reactant gases (metal halide in the first step and ammonia and silicon halide in the second step) in contact with the hot surfaces of the metal or alloy being treated should be less than about 300 mm. of mercury and preferably less than 100 mm. of mercury. Particularly good results are obtained when the combined partial pressure of the reactant gases is'less than 10 mm. of mercury or between about 1 and 10 mm. of mercury.

In the final treatment stage, the molar ratio of ammonia to silicon halide may be varied to vary the rate of deposition. The molar rate of ammonia to silicon halide may vary from 1:1 to 10:1, although preferably the mole percent of ammonia based on silicon halide and ammonia is about 50 to percent.

In carrying out the process it should be noted that the preliminary or precoating step, using silicon halide and nitrogen, forms an important part of the invention. This preliminary treatment is continued until the entire surface of the alloy is coated with an extremely thin, continuous film of the nitride. After a thin, uniform film has been formed on the metals and alloys particularly described hereinabove, it has been discovered that no further deposition of nitride will take place using the gaseous mixture "alloys particularly described herein. precoat obtained by the use of nitrogen and Silicon halide of silicon halide and nitrogen. At this point the nitrogen is turned off and ammonia substituted therefor to build up the deposit to any thickness desired.

When an attempt is made to coat the alloys and metals, particularly described hereinabove, directly by the use of silicon halide and ammonia, it has been discovered thatthe nitride coating does not adhere to the alloy surfaces. The reason for this is not understood.

When silicon halide and nitrogen are employed to provide a thin nitride precoat, followed by deposition of a gaseous admixture of ammonia and silicon halide, the silicon nitride coating tenaciously adheres to the surface of the base metal or alloy, and a continuous uniform J deposit may then be built up.

With a gaseous admixture of nitrogen and silicon halide alone, as has already been pointed out, it is not possible to get a thick deposit of the nitride. The film produced by nitrogen and silicon halide alone does not afford the desired degree of protection to the metals and It is only when the is followed by deposition from a gaseous admixture of silicon halide and ammonia that a tenacious coating sufficiently thick to afford adequate protection against oxida- Example I A pyrolytic silicon nitride coating on a strip of a columbium base alloy consisting essentially of columbium 0.6? x 0.6 x 0.035" was prepared as follows:

Nitrogen and silicon tetrafluoride were separately but simultaneously introduced into one end of a graphite cylinder 6 inches long with an inside diameter of 1.5

sure of 3 mm. Hg. The flow of silicon tetrafluoride was 3.3 millimoles/min. and that of nitrogen was 15.6 millimoles/min. while the temperature and pressure were maintained. After two hours the temperature was reduced to 1000 C., the flow of silicon tetrafluoride was in- 10 sure was raised to 7 mm. Hg.

Example I, except as follows:

temperature.

and about 1% zirconium by weight, and measuring The characteristics of the pyrolytic silicon nitride coating obtained in this example Were substantially the same as those for Example Example 'III g A pyrolytic silicon nitride coating on a strip of a nickel base refractory alloy consisting essentially of nickel, 15.5% by weight chromium, and 8% by weight iron was obtained by the same method as set forth in Treatment with the gaseous mixture of nitrogen and silicon tetrafluoride was at 900 C., and this was followed by treatment with the gaseous mixture of anhydrous ammonia and silicon tetrafluoride at the same The characteristics of the coating obtained were the same as those for the coating of Example I.

Example IV I A pyrolytic silicon nitride coating on a strip of a nickel base refractory alloy consisting essentially of nickel, and in approximate percentage by weight, 19.5% chromium, 4.25% molybdenum, 3% titanium, 1.4% aluminum, .005 boron, and .085% zirconium, was obtained by the same method as set forth in Example I,

except as follows:

inches. The other end of the cylinder was vented to as;

vacuum pump.

Silicon tetrafluoride and nitrogen were introduced into the cylinder at a rate of 3.3 millimoles per minute and 15.6 millimoles per minute, respectively. The cylinder contained'the columbium base alloy strip lying in its center with the plane of the strip parallel to the cylinder axis, and the cylinder was resistively heated to 1010 C. as measured by a pyrometer sighted through a small hole in the cylinder wall. The temperature was raised to 1100 C. after one half hour. An absolute pressure of 2 mm. of mercury was maintained throughout the operation of the'process. the columbium base alloy strip in this manner for several hours, the nitrogen was reduced to 12.2 millimoles/min. and anhydrous ammonia substituted therefor. hour the nitrogen was turned off and the ammonia flow was increased. Feeding of silicon tetrafluoride was continued as before, so that the ammonia and silicon tetrafluoride were separately but simultaneously introduced into the reactor. tetrafluoride were added at the rate of 7.8 millimoles per minute and 3.3 millimoles per minute, respectively. A pyrolytic silicon nitride coating was deposited on the columbium base alloy strip.

The pyrolytic silicon nitride coating on the columbium base alloy'strip thus produced was uniform, translucent, hard, nonporous and chemically inert, and had low thermal conductivity.

Example 11 A pyrolytic silicon nitride coating was obtained on a chrome-plated nickel strip /2" x /2" x 0.03 by the same method as described in Example I, except as follows:

A mixture of silicon tetrafluoride and nitrogen was in- After one In the final stage, the ammonia and silicon The gaseous mixture of nitrogen and silicon tetrafluoride was passed over the sample at 1010 C. and 3 mm. Hg pressure. A flow of 3.3 millimoles/min. silicon tetrafluoride and 15.6 millimoles/rnin. nitrogen was used. After one half hour the pressure was raised to 7 mm. Hg. After three and one half hours anhydrous ammonia was added at a rate of 3.3 millimoles/min. and the flow of nitrogen was reduced to 12.3 millimoles/min. The pressure was reduced to 3-mm. Hg.

One hour later the flow of nitrogen was stopped and After treating the surface of v 6.6 millimoles/min. of silicon tetrafluoride and 15.6 millimoles/min. of anhydrous ammonia was added. The -pressure was raised to 7 mm. Hg.

Example V A pyrolytic silicon nitride coating of a strip of a cobalt hase alloy consisting essentially of cobalt, 21% chromiumby weight, 2% iron by weight, 11% tungsten by weight, and 2% columbium by weight, was obtained by the same method set forth in Example I, exceptas 'drous ammoniaand silicon tetrafluoride was at 1010 C.

In the experiments with this example an attemnt was made'toapply the first treatment at 1010" C. but this failed to yield an adherent coating. An attempt to apply the second treatment at 1100 C. also failed to yield an 55 adherent coating of pyrolytic silicon nitride.

The pyrolytic silicon nitride applied as a coating in f this example had the same characteristics as the coating in Example I. a

It should be understood that the ammonia used in the foregoing example was anhydrous ammonia.

troduced into the cylinder of Example I, containing the chrome-plated nickel strip disposed as in Example I, when the cylinder was at 1100 C. and was under pres Attempts to coat strips identical in composition to those of Examples II, IILiIV and V and a strip of unplated and unalloyed nickel by treatment with silicon tetrafluoride and air followed by treatment with silicon tetrafluoride and ammonia were unsuccessful inall exam ples, except for Example IV on which a discontinuous coating was obtained at 1200 C. When these same metals were treated with hydrogen (saturated with water at room temperature) and silicon tetrafiuoride, all failed to coat except the nickel, which formed a discontinuous coating.

As set forth in the examples, in accordance with this invention, a pyrolytic silicon nitride coating having a resistance to oxidation or corrosion in air at high temperatures well above the maximum operating temperatures of nickel and cobalt base refractory alloys and columbium base alloys created for high temperature use protects these alloys from oxidation and corrosion attack when at their high temperatures of expected use.

In a modified embodiment of the invention for coating nickel and cobalt base refractory alloys, the alloys are first treated with air at about 850 C. to 950' C., preferably about 900 C., to provide an oxide coating on the metals. The alloy is then heated to about 950 C. to lO50 C., preferably about 1000 C., at the reduced pressure indicated hereinabove in a stream of nitrogen containing a metal halide, such as silicon, titanium, or zirconium halide, but preferably titanium tetrachloride. For example, the nitrogen could be saturated with titanium tetrachloride by passing the nitrogen over or through titanium tetrachloride. After a nitride precoat has formed on the alloy, anhydrous ammonia is added to the strearn. Finally, the alloy is treated with anhydrous ammonia and silicon tetrafiuoride, as indicated hereinabove.

Although particularly applicable to nickel base refractory alloys, cobalt base refractory alloys, and columbium base alloys, it should be understood that the invention may also be used, if desired, to treat a wide variety of metals and alloys which are resistant to decomposition at temperatures above 875 C.

The invention in its broader aspects is not limited to the specific details shown and described, but departures may be made from such details within the scope of the accompanying claims without departing from the process of the invention and without sacrificing its chief advantages.

What is claimed is:

1. A method for producing a thin, adherent, nonporous, oxidation and corrosion resistant pyrolytic silicon nitride coating on a base metal that is a member selected from the group consisting of nickel base refractory alloys, cobalt base refractory alloys, and columbium base alloys, which comprises contacting a hot surface of the base metal at a temperature within a first range of between about 875 and 1150 C. with a gaseous mixture comprising metal halide and nitrogen, said metal halide being capable of reacting with nitrogen Within the first temperature range to form a solid metal nitride deposit upon said hot surface, and then contacting the hot surface at a temperature within a second range of between 875 and 1200 C. with a gaseous mixture comprising silicon halide a nd ammonia, said silicon halide being capable of reacting with ammonia to deposit additional silicon nitride on said hot surface; the temperature of contact being controlled within both the first and the second temperature ranges such that the silicon nitride formed adheres to the hot surface during contact of each of said gaseous mixtures with the hot surface.

2. The invention as defined in claim 1, in which the metal halide and silicon halide are both silicon tetrafluoride.

3. The invention as defined in claim 1, wherein the silicon halide and ammonia are introduced separately but simultaneously into the vicinity of the base metal.

4. The invention as defined in claim 1, in which the absolute pressure on the base metal is less than about 300 mm. of mercury.

5. The invention as defined in claim 1, in which the 5 molar ratio of ammonia to silicon halide is between about 1 to l and 10 to l.

6. The invention as defined in claim 1, in which the base metal is a nickel-chromium-iron alloy and in which the temperature is maintained at about 900 C. throughout the method.

7. The invention as defined in claim 1, in which the base metal is a nickel base refractory alloy, and in which the temperature is maintained at about 1010 C. throughout the method.

8. The invention as defined in claim 1, in which the base metal is chrome-plated nickel, and wherein the contacting temperature for the gaseous mixture of nitrogen and metal halide is about 1100 C. and wherein the contacting temperature for the gaseous mixture of ammonia and silicon halide is about 1010 C.

9. The invention as defined in claim 1, in which the base metal is a cobalt base refractory alloy and wherein the contacting temperature for the gaseous mixture of nitrogen and metal halide is about 1100" C., and the contacting temperature for the gaseous mixture of ammonia and silicon halide is about 1010 C.

10. The invention as defined in claim 1, in which the base metal is a columbium base alloy and in which a temperature of about llOO C. is maintained throughout the method.

11. The method of claim 1 wherein the absolute pressure on the base metal is less than about 10 mm. of

mercury.

12. The method of claim 1 wherein the metal halide is a halide of a metal selected from the group consisting of silicon, titanium, and Zirconium.

13. The method of claim ll wherein the base metal is a member selected from the group consisting of nickel and cobalt base refractory alloys, and wherein the hot surface of the alloy contacted with the mixture of metal halide and nitrogen contains an oxide film.

14. The method of claim 13 wherein the oxide film is formed on said hot surface by contacting the hot sur face with air prior to subjecting it to the mixture of metal halide and nitrogen.

15'. A method producing a thin, adherent, nonporous, oxidation and corrosion resistant pyrolytic silicon nitride coating on a base metal that is a member selected from the group consisting of nickel base refractory alloys, cobalt base refractory alloys, and columbium base alloys, which comprises contacting a hot surface of the base metal at a temperature within a range of about 875 to 1150 C. and an absolute pressure of less than about 10 mm. of mercury first with a gaseous mixture comprising silicon tetrafiuoride and nitrogen, and then with a gaseous mixture comprising silcon tetrafiuoride and ammonia, said temperature being controlled such that silcon nitride is adherently deposited upon the hot surface during contact of each of said gaseous mixtures with the hot surface.

16. The method of claim 15 wherein the partial pressure of silicon tetrafiuoride in the first gaseous mixture is less than about 10 mm. of mercury, and wherein the combined partial pressure of silicon tetrafiuoride and ammonia in the second gaseous mixture is less than about 10 mm. of mercury.

17. The improvement of claim 16 wherein the pressure at contact is less than about 10 mm. of mercury absolute.

18. In a method for producing a thin, adherent, nonporous oxidation and corrosion resistant pyrolytic silicon nitride coating on base metals and alloys which are resistant to decomposition at temperatures above about 875 C., the improvement which comprises contacting a surface of the metals and alloys at a temperature above about 875 C. with a first gaseous mixture comprising silicon halide and nitrogen, and then with a second gaseous mixture comprising silicon halide and ammonia, said temperature being controlled such that silicon nitride 1s adherently deposited upon the hot surface during contact of each of said gaseous mixtures with the hot surface.

19. A method of producing a thin, adherent, nonporous, oxidation and corrosion resistant pyrolytic silicon nitride coating on a base metal that is a member selected from the group consisting of nickel base refractory alloys, cobalt base refractory alloys, and columbi'um .base alloys, which comprises contacting a hot surface of .the combined partial pressure of silicon tetrafluoride and ammonia is less than about 300 mm. of mercury, said temperature being controlled such that silicon nitride is adherently deposited upon the hot surface during contact .of each of said gaseous mixtures with the hot surface.

References Cited by the'Exa minerf UNITED STATES PATENTS 448,915 3/91 E'rlwein 117--106 3,019,137 1/62 Hanlet 117-106 3,073,717 1/63 Pyle et a1. 117-69 3,095,527 6/63 Barnes et a1. 117--217 RICHARDD. NEVIUS, Primary Examiner. WILLIAM D. MARTIN, Examiner. 

1. A METHOD FOR PRODUCING A THIN, ADHERENT, NONPOROUS, OXIDATION AND CORROSION RESISTANT PYROLYTIC SILICON NITRIDE COATING ON A BASE METAL THAT IS A MEMBER SELECTED FROM THE GROUP CONSISTING OF NICKEL BASE REFRACTORY ALLOYS, COBALT BASE REFRACTORY ALLOYS, AND COLUMBIUM BASE ALLOYS, WHICH COMPRISES CONTACTING A HOT SURFACE OF THE BASE METAL AT A TEMPERATURE WITHIN A FIRST RANGE OF BETWEEN ABOUT 875* AND 1150*C. WITH A GASEOUS MIXTURE COMPRISING METAL HALIDE AND NITROGEN, SAID METAL HALIDE BEING CAPABLE OF REACTING WITH NITROGEN WITHIN THE FIRST TEMPERATURE RANGE TO FORM A SOLID METAL NITRIDE DEPOSIT UPON SAID HOT SURFACE, AND THEN CONTACTING THE HOT SURFACE AT A TEMPERATURE WITHIN A SECOND RANGE OF BETWEEN 875* AND 1200*C. WITH A GASEOUS MIXTURE COMPRISING SILICON HALIDE AND AMMONIA, SAID SILICON HALIDE BEING CAPABLE OF REACTING WITH AMMONIA TO DEPOSIT ADDITIONAL SILICON NITRIDE ON SAID HOT SURFACE; THE TEMPERATURE OF CONTACT BEING CONTROLLED WITHIN BOTH THE FIRST AND THE SECOND TEMPERATURE RANGES SUCH THAT THE SILICON NITRIDE FORMED ADHERES TO THE HOT SURFACE DURING CONTACT OF EACH OF SAID GASEOUS MIXTURES WITH THE HOT SURFACE.
 13. THE METHOD OF CLAIM 1 WHEREIN THE BASE METAL IS A MEMBER SELECTED FROM THE GROUP CONSISTING OF NICKEL AND COBALT BASE REFRACTORY ALLOYS, AND WHEREIN THE HOT SURFACE OF THE ALLOY CONTACTED WITH THE MIXTURE OF METAL HALIDE AND NITROGEN CONTAINS AN OXIDE FILM.
 14. THE METHOD OF CLAIM 13 WHEREIN THE OXIDE FILM IS FORMED ON SAID HOT SURFACE BY CONTACTING THE HOT SURFACE WITH AIR PRIOR TO SUBJECTING IT TO THE MIXTURE OF METAL HALIDE AND NITROGEN. 